Download ch07 - OoCities

7.3
For each of the following change, indicate whether a confidence interval for  will
become longer or shorter:
a. An increase in the level of confidence
b. An increase in the sample size.
c. A decrease in the level of confidence.
d. A decrease in the sample size.
s
1.
B（error bound or margin of error 誤差界限）= z / 2
n
=half length of C.I.，可以判斷信賴區間的大小
2.
longer; shorter; shorter; longer
7.11
In an article in Accounting and Business Research, Carslaw and Kaplan investigate
factors that influence “audit delay” for firms in New Zealand. Audit delay, which is
defined to be the length of time (in days) from a company’s financial year-end to the
date of the auditor’s report, has been found to affect the market reaction to the report.
This is because late reports seem to often be associated with lower returns and early
reports seem to often be associated with higher returns.
Carslaw and Kaplan investigated audit delay for two kinds of public
companies—owner-controlled and manager-controlled companies. Here a company is
considered to be owner controlled if 30 percent or more of the common stock is
controlled by a single outside investor ( an investor not part of the management group
or board of directors). Otherwise, a company is considered manager controlled. It was
felt that the type of control influences audit delay. To quote Carslaw and Kaplan:
Large external investors, having an acute need for timely information, may be
expected to pressure the company and auditor to start and to complete the audit as
rapidly as practicable.
a. Suppose that a random sample of 100 public owner-controlled companies in New
Zealand is found to give a mean audit delay of x  82.6 days with a standard
deviation of s  32.83 days. Calculate a 95 percent confidence interval for the
population mean audit delay for all public owner-controlled companies in New
Zealand.
s 

1. Large sample mean C.I. =  x  z / 2
n 

32.83
2. 95%C.I .  82.6  1.96 
 76.761,86.035
100
b. Suppose that a random sample of 100 public manager-controlled companies in New
Zealand is found to give a mean audit delay of x  93 days with a standard deviation
of s  37.18 days. Calculate a 95 percent confidence interval for the population mean
audit delay for all public manager-controlled companies in New Zealand.
s 

1. Large sample mean C.I. =  x  z / 2
n 

37.18
2. 95%C.I .  93  1.96 
 85.713,100.287
100
c. Use the confidence intervals you computed in parts a and b to compare the mean
audit delay for all public owner-controlled companies versus that of all public
manager-controlled companies. How do the means compare? Explain.
※ Mean audit delay for public owner-controlled companies
appears to be shorter
7.21
In its October 7, 1991, issue, Fortune magazine reported on the rapid rise of fees and
expenses charged by various types of mutual funds. As stated in the article:
a. Suppose the average annual expense for a random sample of 12 stock funds is 1.63
percent with a standard deviation of 0.31 percent. Calculate a 95 percent confidence
interval for the mean annual expense charged by all stock funds. Assume that stock
fund annual expenses are approximately normally distributed.

s 
1. Small sample mean C.I.=  x  t / 2
  d . f .  11, t11,0.25  2.201
n

0.31
2. 95%C.I .  1.63  2.201 
 1.433,1.827
12
b. Suppose the average annual expense for a random sample of 12 municipal bond
funds is 0.89 percent with a standard deviation of 0.23 percent. Calculate a 95 percent
confidence interval for the mean annual expense charged by all municipal bond funds.
Assume that municipal bond fund expenses are approximately normally distributed.

s 
1. Small sample mean C.I.=  x  t / 2
  d . f .  11, t11,0.25  2.201
n

0.23
2. 95%C.I .  0.89  2.201 
 0.744,1.036
12
c. Use the 95 percent confidence intervals you computed in parts a and b to compare
the average annual expense for stock funds with that for municipal bond funds. How
do the averages compare? Explain.
※ Average annual expenses for stock funds appear to be higher
7.29
Referring to Exercise7.11a(page 267), regard the sample of 100 public owner
controlled companies for which s  32.83 as a preliminary sample. How large a
random sample of public owner-controlled companies is needed to make us
a. 95 percent confident that x , the sample mean audit delay, is within four days of
 , the true mean audit delay?
z 
 1.96  32.83 
※ n  (  / 2 )2  
  258.78  259
B
4


b. 99 percent confident that x is within four days of  ?
2
z 
 2.575  32.83 
※ n  (  / 2 )2  
  446.66  447
B
4


2
7.47
In the book Cases in Finance, Nunnally and Plath present a case in which the estimated
percentage of uncollectible accounts varies with the age of the account. Here the age of an
unpaid account is the number of days elapsed since the invoice date.
Suppose an accountant believes the percentage of accounts that will be uncollectible
increases as the ages of the accounts increase. To test this theory, the accountant randomly
selects 500 accounts with ages between 31 and 60 days from the accounts receivable ledger
dated one year ago. The accountant also randomly selects 500 accounts with ages between
61 and 90 days from the accounts receivable ledger dated one year ago.
a. If 10 of the 500 accounts with ages between 31 and 60 days were eventually classified
as uncollectible, find a point estimate of and a 95 percent confidence interval for the
proportion of all accounts with ages between 31 and 50 days that will be uncollectible.

pˆ (1  pˆ ) 
1. 95%C.I. for population proportion=  pˆ  z / 2

n


0.02  0.98
 0.0077,0.0323
500
b. If 27 of the 500 accounts with ages between 61 and 90 days were eventually classified
as uncollectible, find a point estimate of and a 95 percent confidence interval for the
proportion of all accounts with ages between 61 and 90 days that will be uncollectible.

pˆ (1  pˆ ) 
1. 95%C.I. for population proportion=  pˆ  z / 2

n


2.
95%C.I .  0.02  1.96 
0.054  0.946
 0.034,0.074
500
c. Based on these intervals, is there strong evidence that the percentage of accounts aged
between 61 and 90 days that will be uncollectible is higher than the percentage of
accounts aged between 31 and 60 days that will be uncollectible? Explain.
2.
95%C.I .  0.054  1.96 
1. Yes, because there is no any overlapping at 95% confidence
interval. Evidence level is at 5%, that means strong evidence.